Radiation fog forms during all months of the year but is not a detriment to flying except during late fall, winter and early spring. Due to the nearness of water areas, radiation fog forms only in those areas immediately adjacent to KNPA, west
through northwest. Inland areas to the west and clockwise through
east are ideal formation areas for ground fog. Ground fog formation is most likely to occur with winds of 5-12 knots. Winds of less than 5 knots or greater than 12 knots produce only light ground fog.
Usually a ground fog situation begins 2-3 days after a high or ridge
becomes quasi-stationary over the southeastern United States. The
advected ground fog becomes a factor at KNPA during two wind
1) The scenario with the earliest onset of fog, the most pronounced fog, and the latest fog dissipation is produced by a surface wind from west-northwest clockwise through east-northeast with a speed of 5-12 knots. This primarily occurs during winter, but may occasionally happen during late fall and early spring. IFR conditions due to reduced
visibility begin as early as 2000 local and decrease to below minimums
by 2400 local. IFR conditions remain below minimums until about 1 hour
after sunrise. VFR conditions are observed between 0900 to 1000
local. Boat operations are hampered in all bay areas and as far out
as 3 miles from the coast.
2) A synoptic situation producing a light surface wind can also allow the formation
of weaker ground fog inland. In this situation the land/sea breeze is the predominant wind.
Relatively light winds occur in the period from sunset until early morning. As the land breeze sets in, usually about 0400 local, visibility begins to decrease, becoming
IFR about sunrise. The minimum visibility, 3/4-11/4 mile, occurs
about 1 hour after sunrise. Normal insolation dissipates the
advected fog usually within 2 hours after sunrise. This occurs
mainly during the early fall and late spring.
With conditions favorable for ground fog formation and wind forecast
from the east-southeast clockwise through west-southwest, the ground
fog remains north of the station. Prevailing visibility remains 4-6
miles with quadrants west through northeast occasionally reducing to